Correlative conjunctions always appear in pairs -- you use them to link equivalent sentence elements. The most common correlative
conjunctions are "both...and," "either...or," "neither...nor,", "not only...but also," "so...as," and "whether...or." (Technically
correlative conjunctions consist simply of a co-ordinating conjunction linked to an adjective or adverb.)

The highlighted words in the following sentences are correlative conjunctions:

Both my grandfather and my father worked in the steel plant.

In this sentence, the correlative conjunction "both...and" is used to link the two noun phrases that act as the compound subject of the sentence: "my grandfather" and "my father".